The present invention relates generally to moisture measuring apparatus, and more specifically to such apparatus in which moisture containing material is confined to flow through a chute wherein the moisture content of the material is determined in a non-destructive manner using microwaves.
It is well known in the art that the moisture contained in a product can be determined by utilizing microwaves which when propagated through the product will be attenuated to an extent which is dependent on the quantity of moisture in the product. This attenuation of microwaves is then converted into water density units. It is also known that if this microwave "bombardment" of the product is combined with nuclear energy (e.g., gamma rays) "bombardment" thereof, a direct percent moisture measurement of the product is obtainable. The attenuation of nuclear energy is proportional to the mass of the material between a nuclear energy source and detector, such that in an enclosed space, such as a chute, completely filled with the product, the attenuation can be converted to density (mass/unit volume). By directing the microwaves and nuclear energy through the same cross-section of the product, the water density can be divided by the mass density to give the direct percent moisture measurement. That such methods for measuring moisture content are well known is evidenced by U.S. Pat. No. 3,460,030 which is hereby incorporated by reference to the extent necessary to complete this disclosure.
The use of microwaves to measure moisture content, however, has inherent problems. For example, when the microwave beam is directed perpendicularly to a product as it moves between microwave transmitter and receiver horns, microwaves are reflected between the horns, causing standing wave patterns and resulting in inaccurate moisture measurements. Also for example, a portion of microwaves in the product will be "scattered" so as to miss the receiving horn, likewise resulting in inaccuracies. When the product is confined in a chute, additional accuracy problems arise from microwaves travelling axially along the chute.
In U.S. Pat. No. 3,818,333 to Walker various modifications to a conventional chute and microwave antenna system are proposed for overcoming the noted inherent problems related to microwave moisture measurements. According to that reference, the walls of the chute are sloped to avoid microwave reflections from the chute walls into the microwave antennas. To prevent refraction of microwaves passing through the microwave antenna windows, the windows have flat end portions which are mutually parallel. This arrangement of the windows also prevents transmissions of microwaves along the walls of the chute. Since the chute walls are sloped, the windows naturally extend into the chute. So that microwave reflections from the inner and outer surfaces of the windows will cancel each other out, the end portions thereof have uniform thicknesses equal to one-half the wavelength of the microwaves. Since the antenna housings are connected to the walls of the chute, microwave absorbent material surrounds the antennas apparently to prevent the launching of surface microwaves into the chute via the connections between the housings and walls. Finally, to prevent reflection of microwaves through the windows from the antenna housing portions located directly behind the antennas, those housing portions are lined with microwave absorbent material.